Many types of retroreflective articles are known, and are made in a variety of ways. One common type of retroreflective article uses transparent microspheres, typically with hemispheric retroreflectors thereon. Examples of this type of retroreflector are disclosed in U.S. Pat. No. 2,407,680 (Palmquist), U.S. Pat. No. 3,190,178 (McKenzie), and U.S. Pat. No. 4,025,159 (McGrath).
Another type of retroreflective article includes prismatic designs incorporating one or more structures commonly known as cube corners. Retroreflective sheeting which employs cube corner type reflective elements is well known. An example of such designs is shown in U.S. Pat. No. 3,684,348 (Rowland).
The manufacture of retroreflective cube corner element arrays is accomplished using molds made by different techniques, including those known as pin bundling and direct machining. Molds manufactured using pin bundling are made by assembling together individual pins which each have an end portion shaped with features of a cube corner retroreflective element. For example, certain pin bundled arrays permit elaborate assembly into various pin structural configurations but these types of arrays become considerably more difficult to manufacture as smaller microcube structures are formed. U.S. Pat. No. 3,926,402 (Heenan et al) and U.S. Pat. No. 3,632,695 (Howell) are examples of pin bundling.
The direct machining technique, also known generally as ruling, comprises cutting portions of a substrate to create a pattern of grooves which intersect to form cube corner elements. The grooved substrate is referred to as a master mold from which a series of impressions, i.e. replicas, may be formed. In some instances, the master is useful as a retroreflective article, however replicas, including multi-generational replicas, are more commonly used as the retroreflective article. Direct machining is an excellent method for manufacturing master molds for small micro-cube arrays. Small micro-cube arrays are particularly beneficial for producing thin replica arrays with improved flexibility, such as continuous rolled goods for sheeting purposes. Micro-cube arrays are also more conducive to continuous process manufacturing. The process of manufacturing large arrays is also relatively easier using direct machining methods rather than other techniques. One example of direct machining is shown in U.S. Pat. No. 4,588,258 (Hoopman).
Some retroreflective articles comprise a ridge-like structure. In the pin bundled field, these types of structures are shown in U.S. Pat. Nos. 4,243,618 (Van Arnam), 4,202,600 (Burke et al), 4,726,706 (Attar), 4,208,090 (Heenan), 4,498,733 (Flanagan), 3,922,065 (Schultz), 3,417,959 (Schultz), and 3,924,929 (Holmen). Another ridge-like structure in a retroreflective article is taught, primarily, for a microsphere or beaded sheeting construction, in U.S. Pat. No. 4,025,159 (McGrath).
Other structures similar to the ridge-like structures described above are described in U.S. Pat. Nos. 4,801,193 (Martin), 4,618,518 (Pricone), and 5,171,624 (Walter). However, these patents merely disclose ridge-like structures which are formed independent of the formation of the adjacent cubes in the retroreflective sheeting. Typically, these structures are cast onto the sheeting rather than being formed integrally, at the same time as the cubes, by direct machining and replication.